In conjunction with the control of the induction motor, there are known a V/f control for controlling the rotation speed of the motor by changing a motor- applied voltage V and a frequency f in a predetermined ratio and a vector control for controlling the induction motor in a manner similar to the control of a DC motor by decomposing the motor current into a magnetic flux corresponding or equivalent component (referred to as d-axis current) and a torque equivalent component (referred to as q-axis current). Heretofore, the first mentioned V/f control has been adopted widely because of simplicity of the control scheme. On the other hand, the second mentioned vector control has found practical applications in place of the conventional DC servo control because of capability of arithmetic operations involved in the control on a real time basis owing to advancement of microcomputer technology.
With the first-mentioned V/f control, torque is unavailable in a low speed region due to a voltage drop brought about by resistance of the armature while encountering difficulty with regard to the positioning. Such being the circumstances, there has been adopted a DC braking for applying brake by causing a DC current to flow through the induction motor.
On the other hand, with the second mentioned vector control, there can be ensured a sufficient magnitude of torque even in the low speed region, and the positioning can equally be realized. However, in the vector control, speed (position) information is required. For this reason, the induction motor has to be equipped with a position sensor such as an encoder for detecting the speed of the induction motor or a speed estimating device for estimating the motor speed on the basis of current and voltage. For coping with failure occurring in these speed detecting means, the aforementioned DC braking is adopted in many cases for realizing emergency stoppage of the induction motor.
Thus, in many practical applications, the second mentioned vector control apparatus is operated similarly to the V/f control type induction motor control apparatus in the DC braking mode by resorting to the same control procedure as the conventional one. Parenthetically, the procedure for controlling the DC braking resides simply in applying to the induction motor a DC voltage corresponding to a desired DC braking force.
FIG. 4 is a block diagram showing a hitherto known or conventional vector control apparatus imparted with a DC braking function. As can be seen in FIG. 4, in the conventional vector control apparatus, there are provided a DC braking control arithmetic unit 1 and switches S1 and S2 in order to realize the DC braking function in addition to arithmetic units 2 to 12 provided inherently for the vector control. In the vector control state, the switches S1 and S2 are closed to the position a while in the DC braking mode, the switches S1 and S2 are closed to the position b. By setting the switch S2 to the position b, the output of a d,q-axis current controller 4 is rendered ineffective, whereby one voltage (vd*) is set to an input value corresponding to a braking force while the other voltage (vq*) being set to zero, as a result of which a DC voltage is applied to an induction motor 13. Further, in order to suppress change-over shock, the switch S1 is so operated as to allow the DC braking control arithmetic unit 1 to arithmetically determine the voltage phase .theta.v which coincides with the d-axis.
The vector control apparatus may be used for realizing servo function such as positioning, as occasion requires. Accordingly, the vector control apparatus is inhibited from stopping upon occurrence of overcurrent or the like event. Same holds true even in the case where the DC braking mode of the vector control apparatus is validated in an emergency. Since the vector control apparatus can ensure sufficient torque even in a low speed region and is capable of performing positioning operation, the DC braking is not required intrinsically. However, when taking into consideration the failure of the position (speed) sensor mentioned previously, the DC braking mode is considered as an indispensable control mode for the vector control apparatus.
At this juncture, it should be mentioned that when the conventional DC braking procedure is adopted intactly, there may arise such a situation in which the apparatus is tripped for protecting itself against overcurrent which may occur upon change-over of the operation mode from the vector control to the DC braking mode. Such trip is likely to take place when the rotation speed is large with the current being easy to increase or when effective load is heavy, because sufficient current control is impossible only with the application of DC voltage as in the case of the conventional DC braking. Since in the vector control apparatus, the DC braking is validated upon occurrence of failure, as described above, it is impossible to limit the condition for validating the DC braking to such a load state where the trip is difficult to occur. Thus, there exists a demand for a DC braking procedure which does not incur the trip in any state.
An object of the present invention is to provide a trip-insusceptible DC braking procedure incarnated in a vector control apparatus of high performance. Further, a second object of the present invention is to provide a DC braking procedure which incurs less vibration upon stoppage.